Heat exposure promotes browning of white fat, too.

The color of fat tissue – white, brown, or beige – determines the role the tissue plays in the body. Whereas white fat is involved primarily in lipid storage, brown fat is involved primarily in heat production. Beige fat, which is co-located with white fat, can adopt either storage or heat-producing properties. Cold exposure, which induces hypothermia, causes white fat to convert to beige fat, a process known as “browning” (or “beiging”). Conversely, findings from a recent study suggest that heat exposure, which induces hyperthermia, also promotes browning of white fat.

Hyperthermia is an increase in the body’s core temperature that induces a thermoregulatory response involving neuroendocrine, cardiovascular, and cytoprotective mechanisms. These mechanisms work together to restore homeostasis and condition the body for future heat stressors, a phenomenon known as hormesis. Evidence suggests that hyperthermia is beneficial against metabolic diseases, such as type 2 diabetes and obesity.

The investigators used a photothermal gel that can convert the energy of near-infrared light to heat to induce local hyperthermia. They injected the gel into both sides of the fatty tissues in the groin area of obese mice and applied near-infrared light to one side of the injected area but not the other. The area that received the near-infrared light exposure warmed to 41°C (~5°C higher than normal body temperature for mice), and heat production continued in the exposed area for 12 hours. Then the investigators applied heat to the neck and shoulders of humans, warming the area to 41°C, and found that the tissue continued to produce heat for two hours.

Next, the investigators gauged the effects of local hyperthermia on metabolism. They found that the mice that received the local hyperthermia treatment were thinner, had better insulin sensitivity, and had fewer fat deposits in their livers than the mice that didn’t receive local hyperthermia.

These findings suggest that local hyperthermia treatment induces browning and heat production in white fat in both mice and humans and improves metabolic function in mice, providing a potential means for treating obesity. Learn more about the beneficial effects of hyperthermia in our overview article.

• Link to study abstract.